U.S. Pat. Nos. 3,293,096, Nairn et al, 3,293,108, Nairn et al, and 2,920,977, Adams, describe products and processes for producing webs by applying a chemically foamable plastisol of polyvinyl chloride resin (PVC) to a substrate such as a web of felted cellulose or mineral fibers. The plastisol is caused to "gel", that is to effect partial solvation of the resin to at least the elastomeric point whereby it becomes like a solid for purposes of subsequent handling. This is accomplished by the application of heat sufficient to achieve temperatures above the gelling temperature of the plastisol but below the temperature at which expansion will occur. A decoration is applied to the web so formed by, for example, rotogravure printing a design onto the surface of the gelled plastisol. A wearing surface is then added to the composite web over the decoration. The wearing surface may be a clear, nonfoamable polyvinyl chloride resinous composition applied in the form of a plastisol. The composite web is then heated to a temperature that causes the foamable plastisol to expand and fuse and the wearing surface or wear layer to solidify and fuse.
In a principal embodiment of the invention described in the Nairn et al patents referred to above, a chemical described as an "inhibitor" is incorporated in one or more of the printing inks applied to the surface of the gelled, expandable plastisol during printing. The inhibitor reduces the amount of subsequent expansion of the gelled, expandable plastisol during the final heating step only in the areas where it is applied. This results in a surface that is fully expanded in all portions except where the inhibitor is applied. A three dimensional or embossed appearance is achieved. The embossing is in perfect register with the color of the printing ink with which the inhibitor is applied.
Products made as described above have achieved world wide commercial success. They have been available in the form of continuous webs ranging in width from six to fifteen feet and lengths determined by convenience of packaging and expected useage. There has not, however, been widespread production of such products in the form of tiles, e.g. squares measureing 9.times.9 inches or 12.times.12 inches, even though there is a significant market for tile products. An important reason for this is that, among other things, the construction of products manufactured by the processes described above introduces inherent problems of dimensional instability caused by the growth or shrinkage of the various layers of the composite product after the product has been produced. As an example overall curling can occur because of shrinkage of certain compositions of wear layers when applied over relatively thick layers of expanded resinous compositions. On the other hand many substrates made from cellulose or mineral fibers exhibit growth when exposed to certain moisture conditions which are somewhat high but occur frequently in normal use. Special installation procedures, normally carried out by professional installation mechanics, overcome some of these problems when the product is used in sheet form. For example, adhesive is applied to the surface to be covered as a separate step prior to the installation of the surface covering. As soon as possible after installation another step is preferably performed at the seams that have been created by abutting various pieces of the sheet product as required to completely cover a defined area. The seams are fused together by the careful application of a solvent for the resinous composition along the entire length of each seam. The proper use of the correct adhesive and seam sealing material results in a finished surface that will not curl and has seams that will not shrink open and become unattractive or hazardous. Normally the areas treated by seam sealing materials are visible. It is obvious that this procedure would be acceptable and impractical for an area to be covered by numerous relatively small tiles.
Substrates different from those described above including substrates based on glass fibers have been used to produce surface covering products. The glass fibers are provided in nonwoven web form prepared by a wet laydown process using a small quantity of a resinous binder to secure the fibers in place. U.S. Pat. No. 4,138,521, Brown, describes glass fiber webs used in the surface covering industry and also describes floor covering products using such webs. One of the products comprises a glass fiber web impregnated with a gelled PVC plastisol for use as a substrate. An expandable or foamable PVC plastisol is applied over the impregnated web and gelled or cured to a solid condition. The composite is subsequently decorated and a wearing surface applied. The sheet is heated to expand and fuse the expandable layer and to fuse the wear layer. In another form a foam layer is additionally applied to the side of the product that is intended to contact the floor.
Although products made with fiber glass substrates instead of cellulose or mineral fiber substrates are less affected by dimensional changes due to the substrate, the resinous compositions used in other layers of the products are subject to dimensional changes. For example wear layer shrinkage can occur upon exposure to normal atmospheric conditions over a period of time. Certain conditions such as high heat will accelerate shrinkage. Formulation and processing techniques known in the art are used to minimize this problem in the construction of surface covering products.
Shrinkage of the top layer of a tile product is more critical than shrinkage of this layer, which is the wearing surface, on a product produced in sheet form. Wear layer shrinkage on a tile product can cause individual tiles to curl out of an adhesive used to bond it to a floor. It can cause the development of open seams around each tile which become recepticles for soil and tripping hazards.
A wear layer adhered directly to a PVC impregnated fiberglass substrate may have good dimensional stability; however, both curl and shrinkage are adversely affected by interposing a foam layer between this substrate and a wear layer. The closer the wear layer is to a properly formulated and processed fiber glass substrate the better anchor the substrate becomes for resisting the inherent tendency of the wear layer to shrink and cause the product to curl. Experience has indicated that the presence of no more than about 0.025 inches in thickness of an expanded PVC foam layer over a PVC impregnated fiber glass substrate is practical in the construction of floor covering products in tile form. In this range formulations can be designed for minimum curl and shrinkage yet enough foam thickness is present to achieve adequate depth of embossing by chemical or mechanical means in those cases where embossing is important to the desired decorative effect. For any given thickness of an expanded or foamed resinous layer, the thicker the wearlayer on top of it, the more pronounced is the tendency to curl and to shrink. On the other hand the combination of a thick expanded resinous layer and a thin wear layer generally results in an unsatisfactory product. The latter combination aggravates the ease with which the product can be punctured by sharp objects or worn through by abrasion.
Surface coverings in tile form that are produced giving recognition to the above constraints are by necessity relatively thin, e.g. some products are constructed with a PVC impregnated fiber glass web of about 0.020 inches in thickness, a foam interlayer of about 0.025 inches, and a wear layer of about 0.007 inches, for a total thickness of about 0.052 inches. The usefullness of such products is severely limited because sufficient material is not present to achieve the structural integrity necessary for many applications. Small irregularities in the subsurface to which the surface covering is applied will show through to the surface. Some irregularities in the subsurface can burst through the surface covering from the pressure exerted in normal use. The top surface is easily punctured from above. Whether in tile or sheet form the products have poor "hand", that is, they are limp and lack the stiffness characteristics that are often desired for surface covering products. These products have especially poor hand when compared to surface covering products directed to use as floor covering but constructed in a more conventional manner. Such products are severely limited in their ability to offer controlled insulating properties or controlled resiliency when used as floor coverings. Tile and sheet products constructed as above are commercially available.
Added thickness can be achieved by placing a PVC resinous composition layer on the back of a product produced with a resinous impregnated fiber glass substrate. This layer may be nonexpanded or it may be expanded as shown in the aforementioned Brown patent. An added layer of expanded composition does little to improve dimensional stability in use. The back foam layer appears to be "working" or continuously moving when exposed to the forces that are experienced in normal use. After a period of time the constant repetition of compression and decompression and the action of the shearing forces causes the product to become loose in spots on the floor. The problem is aggravated at or near seams. Special adhesives are recommended for the installation of products in sheet form haveing a foam layer as the back of the product.
A nonexpanded resinous back layer creates additional problems because of the added mass having inherent tendencies of curl, growth or shrinkage. Both the expanded and nonexpanded back layer help resist show through of subfloor irregularities to some degree. With time however subfloor irregularities begin to show through because of the thermoplastic nature, and thus the maleability, of the back layer of material. Products of this type also have unique adhesion problems and special adhesives are normally required to achieve bonding to a subsurface.
Webs that have thermoplastic surfaces on both sides of an interlayer create special operating problems when it is required to expose the web to high temperatures. Various costly systems must be used to avoid disturbing either the top or the back when heating to achieve gelling and fusion. The use of release paper to carry the web during exposure to heat is one approach that is known in the art as is the use of specially designed "floater" ovens in which a web is processed on a layer or conveyer of air rather than rollers or metal or fabric conveyers.